1. Field of the Invention
The present invention relates generally to polymers, and more particularly to organometallic initiators capable of polymerizing and copolymerizing olefins.
2. Related Art
Interest in metal-mediated polymerization of olefins remains unabated in academic and industrial laboratories. Recent advances in stereo control and improved functionality tolerance, together with insight from mechanistic and theoretical studies, have considerably improved control over the final polymer structures and thereby the bulk properties of the resulting materials (1, 2, 3, 4, 5, 6, 7).
Tailoring of the bulk properties of polyolefins by synthetic control is still a major focus of many industrial and academic research groups. These efforts have produced a progression in catalysts design, from heterogeneous catalysts to “single-site” systems whose reactivity can be fine tuned by choice of the ligand environment surrounding the active metal center (1, 2, 3, 4, 5, 6, 7). Novel structures based on electrophilic early transition metal based catalysts, together with advances in the activation by co-catalysts and improved mechanistic understanding, have played a significant role in this development. Examples of how the metal center can tailor properties by controlling monomer insertion include the stereo specific polymerization of propene (8, 9, 10, 11, 12, 13, 14) and the copolymerization of ethylene and 1-alkenes (15, 16, 17, 18).
Recently the design of late transition metal initiators has received renewed interest because of their lower oxophilicity and resistance toward deactivation by polar functionalities, relative to their early transition metal counterparts (19, 20, 21, 22, 23). This reduced sensitivity of late metals to polar impurities allows for polymerizations to be carried out under less stringent conditions and allows for the copolymerization with polar comonomers (24, 25, 26, 27, 28). Nickel and palladium based catalysts have been shown to participate in chain walking reactions (29, 30, 31), tolerate polar functionalities (24, 25, 26, 27, 28) and have even been used in water (32, 33, 34). These catalytic properties are of significant interest for developing materials with unique properties and for the development of new commercial processes.
The introduction of cationic Pd(II) and Ni(II)-based catalysts which convert ethylene and α-olefins to high molar mass polymers, by Brookhart et al., rejuvenated the area of ethylene polymerization with late metal catalysts (1, 2, 3, 4, 5, 6, 7, 19, 20). The highly electrophilic metal center, and steric bulk on the ligand are important features for the generation of high molecular weight polymer. The electrophilicity of the late metal center results in rapid rates of olefin insertion while the use of bulk favors insertion over chain transfer. The variation of the backbone and aryl substituents, on the ligand, allow for further control over steric and electronic effects at the metal center (1, 2, 3, 4, 5, 6, 7, 46).
Copper(II) and zinc(II) complexes supported by a β-diimine ligand, undergo oxidative degradation to give a ketone diimine derivative under aerobic conditions (Yokota et al.) (44).
Recent literature shows current interest in the transition metal mediated living polymerization of olefins. These reactions allow for the synthesis of polyolefins with higher order architectures and improved physical properties (47, 48). In contrast to early transition metal catalysts, late metal systems are more tolerant toward functionality (49, 20, 21, 50) and participate in “chain-walking” reactions (3, 30, 51, 52, 53) in which the metal center migrates along the growing polymer chain through a series of β-hydride elimination and reinsertion steps. The polymerization of ethylene by late metal cationic systems results in polyethylene (PE) with various degrees of branching. Similarly, “ethylene” sequences (18, 54) can be generated from the chain straightening of higher α-olefins (3, 30, 51, 52, 53). These distinctive features in combination with living behavior, have led to the generation of novel materials such as elastomeric multi-block poly(α-olefins) (54), ethylene-propylene type copolymers (18), regioblock copolymers (18, 55, 56), and end-functionalized amorphous PE (24).